Sound recording and reproducing



G. L. DIMMICK 2,184,547

SOUND RECORDING AND REPRODUCING Filed Feb. 27, 1937 2 Sheets-Sheet 1Snventor QQWZJM W (Ittomeg Patented Dec. 26, 1939 UNITED STATES PATENTOFFICE Glenn L. Dimmick, Erlton, N. J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application February 27, 1937, SerialNo. 128,072

8 Claims.

This invention relates to an improvement in a method of and an apparatusfor recording and reproducing sound, and, more particularly, involvingthe recording and reproduction of sound on film. The purpose of theinvention is to provide improved response at low frequencies and at thesame time to reduce ground noise.

It has heretofore been proposed, as is described and claimed in KarolusPatents 1,901,033 and 10 1,901,034, issued March 14, 1933, to record andreproduce the high frequency components of sound with a very narrow lineof light, omitting the low frequency components from that record. Thelow frequency components, which do not require such high resolvingpower, were recorded in a separate record by a much wider beam of light,thereby permitting increased exposure in recording, and in reproducingthe low frequencies were reproduced by means of a correspondingly widebeam of light, thereby giving greater photocell response. This systemhad the disadvantage that two separate sound-tracks were required, forwhich space was not readily available on the ordinary sound picturefilm.

'25 In the performance of my invention I record both the high and lowfrequencies by means of a relatively narrow beam of light but with anappropriately low modulation in the low frequencies, In reproducingsound from this record I then scan the high frequencies by means of arelatively narrow beam of light havingadequate resolving power and scanthe low frequencies by means of a relatively wide beam of light coaxialwith the narrow beam of light but which, due'to its width, picks uprelatively less ground noise from the film.

My reproducing apparatus is further so constructed as to permit theselective reproduction of my improved sound record or ordinary variabledensity or variable area sound records or push-' pull sound records, theapparatus being set for either type of record by merely turning aswitch.

One object of my invention is to provide an 45, improved sound record.

Another object of my invention is to provide an improved method ofrecording sound.

Another object of my invention is to provide an improved method ofreproducing sound. 50 Another object of my invention is to provide animproved sound reproducing apparatus.

Another object of my invention is to provide sound reproducing apparatushaving an improved frequency response. 55 Another object of my inventionis to provide sound reproducing apparatus producing a minimum of groundnoise.

Another object of my invention is to provide a sound reproducingapparatus readily adaptable to the reproduction of any commercial type 5of sound record.

Other and incidental objects of my invention will be apparent to thoseskilled in the art from a reading of the following specification and aninspection of the accompanying drawings, in 10 which:

Figure 1 is a diagrammatic illustration of a sound reproducer made inaccordance with my invention, together with a schematic diagram of theoutput circuit therefor,

Figure 2 is a detail of the sound reproducing apparatus,

Figure ,3 is a diagrammatic illustration of the manner in which thecoaxial light beams scan the sound record, and 20 Figure 4 illustratesthe frequency characteristics of the sound record and-of the reproducingapparatus.

Referring first to Fig. 1, the customary exciter lamp I0 is provided forilluminating the portion of the sound record to be reproduced. Anoptical system H is provided of the customary opticalslit type whichilluminates a transverse line on the sound track on the film l2 of awidth corresponding to the resolution of-the lowest frequencies, i. e.,the width of the line of light on the sound track is not more thanone-half of the Wave length of the higher end of the low frequency bandto be reproduced. A lens system I3 is provided which, operating inaccordance with Berglund Patent No. 1,564,509, issued December 8, 1925,directs an image of the sound track from the film l2 onto the member I4.This member I4 is provided with a narrow slit 1 5, as in the saidBerglung patent, through which light is trans- 40 mitted to the cathodel6 of a photocell 11. This slit I5 is made of awidth corresponding tonot more than half the wave length of the image of the highest frequencysound waves which are to be reproduced, and these high frequency soundwaves are, accordingly, picked up by the cathode l6 and converted intoelectrical energy'which is fed into the primary winding l8 of atransformer, the secondary 19 of which feeds the current to anappropriate amplifier or other output device.- A high-pass filter 9 maybe inserted in the circuit if desired. The member I i has a mirrorsurface and the portion of the light beam focused thereon by the lenssystem I3, but which is not transmitted by the slit I5, is reflected tothe mirror 20 which in turn directs it on the cathode 2! of thephotocell. Due to the width of this beam of light it is not properlyresponsive to high frequencies nor to much ground noise, but due to thequantity of light transmitted a high degree of modulation is secured onthe lower frequencies. These low frequency impulses are transmittedthrough the filter 22 (which diagrammatically illustrates any desiredtype of low pass filter) which serves to eliminate any residual highfrequency, photocell hiss or ground noise from the current fed to thetransformer primary 23 which feeds current through the transformer tothe secondary iii in the same manner as does the primary l8.

It will be apparent from the foregoing description that high frequenciesare reproduced through the cathode i5 and the lower frequencies throughthe cathode 2i and these are fed in proper phase relation into thetransformer from which they are taken off by the winding ii]. Thefrequency characteristic of this reproducing apparatus is illustrated bythe curve 36, Fig. 4, showing that the apparatus has a very highresponse at frequencies up to a few hundred cycles per second, fallingoff at a frequency determined by the width of the scanning beam on thefilm l2 and having a considerably lower response at the high frequencyend which is scanned by the slit l5. In recording, therefore, it isnecessary to provide a record having a frequency characteristiccorresponding to the curve 3!, which may be readily accomplished by theuse of appropriate high pass filters in the recording amplifier, andwhich has a very high amplitude as indicated at 32 in the higherfrequencies. In this type of record, since the amplitude of a modulationof the high frequencies is relatively very high, the ratio of sound toground noise is correspondingly high in these frequencies, while at thelower frequencies the ground noise is not reproduced by the apparatusand the modulation can be correspondingly low. In the reproduction of asound record according to the curve 3i by means of my apparatus having aresponse characteristic corresponding to the curve 30 a substantiallyfiat overall response is obtained.

The relation of a low frequency sound wave to my scanning beam isillustrated diagrammatically in Fig. 3 where l5 indicates the areacovered by the scanning slit [5, ll indicates the area scanned by theoptical system H, and 62 shows a variable area sound record as it mightappear on the film I2. It will be apparent that the beam H canadequately scan the low frequency waves indicated and at the same timeproduces a very large response, while the beam l5 which is capable ofscanning the higher frequencies does not transmit as much light.

In order to render the apparatus generally useful, and not limit itsusage to the specific type of record just described, I provide thecircuit shown in Fig. 1 with an appropriate changeover switch forshifting the device to any desired type of record.

This changeover switch includes three threeway switches with a singleactuating means therefor as indicated in Fig. 1, and serves to so changethe circuits as to permit selectively the use of either my improvedcircuit or an appropriate circuit for pushpull records or a circuit forstandard film, as described and claimed in Belar application, Serial No.52,458, filed December 2, 1935, one form of which is illustrated inFig. 1. In the left hand position of the switch blade 24 the upper endof transformer winding 23 is connected to the battery 21 while the lowerend of the winding 23 is connected through the blade 25 to the cathode2! through the filter circuit 22. The winding 23 is therefore operatingin the same direction as the winding I8 and in accordance with myimproved method above described. In the middle position of the blades 26and 25, the cathode 2! is connected to the upper end of the winding 23omitting the filter circuit 22 and the lower end of winding 23 isconnected to the battery 2'5, thereby permitting the two cathodes tooperate in pushpull relation. In the third position of these switches,the oathode 21 is again connected to the winding 23 as in the firstposition, but the filter circuit 22 is omitted and the apparatus is thenin condition to reproduce ordinary sound records.

The switch blade 26 is used to control the slit plate shown in detail inFig. 2. This slit plate has the reflecting surface 94 and the slit 15 asdescribed in connection with Fig. 1 and as shown in operative positionin Fig. 2. This slit plate is carried in appropriate frame 33 slidablein a second frame 34 and provided with stop rods 35 and 36 to the latterof which it is connected by the rod 31. A spring catch 38 is providedfor retaining the frame 33 in either of two positions. In the positionshown, light passes through the slit it to the cathode l6 and anotherportion of the beam is reflected from the surface M to the cathode 2! asbefore described. When, however, the frame 33 is moved toward the leftin Fig. 2 the other half of the apparatus comes into use and in thisportion of the apparatus non-reflecting surfaces 59 and at are providedon opposite sides of the slit M, which permits light from one-half ofthe soundtrack to pass through the slit it to the cathode It. This slitH extends only one-half of the length of the surfaces 38 and 4-0 and theother half of their length a narrow mirror 52 is provided. This mirrorhas a width corresponding to that of the slit H and reflects acorrespondingly narrow line of light to the mirror 20 whence it isreflected to the cathode 2!. It will be apparent that when thenon-reflecting portion of the slit plate 39 and id is in operatingposition light from one-half of the sound track will be reflected fromthe mirror 12 while light from the other half is transmitted through theslit 4|, thus causing each cathode to respond to the impulses of onlyone-half of the soundtrack.

The frame 33 is connected by a rod 43 with an armature M adapted toslide back and forth in the windings t5 and 46. When the winding 45 isenergized the frame 33 is moved to the left of Fig. 2, thus bringing theright hand portion of the device into use; and when again the solenoid45 is energized, the slit l5 and mirror M are again brought into use.These solenoids are actuated from an appropriate source of power 47,Fig. 1, by the switch 26, which is interconnected with the switches 24and 25. It will be apparent that the narrow scanning slit and mirror areused both in pushpull reproduction and in the reproduction of ordinaryfilm, while the mirror Ht is automatically shifted into place forreproduction according to my improved method.

It will be apparent to those skilled in the art that my invention is notlimited to the specific embodiment thereof shown in the drawings butthat this may be considerably modified. For example, it is sometimesdesirable to reproduce sounds of low frequency by means of oneloudspeaker or set of loudspeakers and to reproduce the sounds of highfrequency by means of another speaker or set of speakers. In such cases,there is occasionally dimculty due to phase differences between the highand low frequency sounds and I can compensate for this by slightlymoving the image H produced by the optical system H in relation to theslit l5 and this may be accomplished either by moving the optical systemI i, the optical system l3 or the mirror l4.

Likewise, if I so desire I may transmit the high and low frequenciesthrough separate amplifiers, or even through separate amplifiers toseparate loudspeakers.

Having now described my invention, I claim:

1. The method of sound reproduction comprising the steps of scanning animage of the sound track in a narrow line to reproduce high frequenciesand simultaneously scanning an image of the same sound track area in awider line to reproduce low frequencies.

2. The method of sound reproduction comprising the steps of scanning animage of the sound track in a narrow line to reproduce high frequenciesand simultaneously scanning an image of the same soundtrack area in awider line to reproduce low frequencies, and adjusting the relativeposition of said lines to determine the phase relation of the reproducedsounds.

3. Apparatus of the class described comprising means for illuminating asound track with a relatively wide line of light for reproducing lowfrequencies, and means for selecting a narrow portion of the illuminatedarea for reproducing high frequencies.

4. Apparatus of the class described comprising means for illuminating asound track with a relatively wide line of light for reproducing lowfrequencies, means for selecting a narrow portion of the illuminatedarea for reproducing high frequencies, means for directing light fromsaid narrow area to one photoelectric device, and means for directinglight from said illuminated area to another photoelectric device.

5. Apparatus of the class described comprising means for illuminating asound track with a relatively wide line of light for reproducing lowfrequencies, means for selecting a narrow portion means for directinglight from said illuminated area to another photoelectric device, saidphotoelectric devices being connected to a common output circuit.

6. Apparatus of the class described comprising means for illuminating asound track with a relatively wide line of light for reproducing lowfrequencies, means for selecting a narrow portion of the illuminatedarea for reproducing high frequencies, means for directing light fromsaid narrow area to one photoelectric device, and means for directinglight from said illuminated area to another photoelectric device, thefirst of said photoelectric devices being connected directly to anoutput circuit and the second of said photoelectric devices beingconnected to the output circuit through a low pass filter.

7. Apparatus of the class described comprising means for illuminating asound track with a relatively wide line of light for reproducing lowfrequencies, means for selecting a narrow portion of the illuminatedarea for reproducing high frequencies, means for directing light fromsaid narrow area to one photoelectric device, and means for directinglight from said illuminated area to another photoelectric device, thefirst of said photoelectric devices being connected to an output circuitthrough a high-pass filter and the second of said photo-electric devicesbeing connected to the output circuit through a low pass filter.

8. Apparatus of the class described comprising means for illuminating asound track with a relatively wide line of light for reproducing lowfrequencies, means for selecting a narrow portion of the illuminatedarea for reproducing high frequencies, means for directing light fromsaid narrow area to one photoelectric device, means for directing lightfrom said illuminated area to another photoelectric device, saidphotoelectric devices being connected to a common output circuit, andmeans for selectively connecting said photoelectric devices to the saidoutput circuit either in tandem relation or in pushpull relation.

GLENN L. DIIVIMICK.

